class Control {
  private state: any;
}

interface SelectableControl extends Control { // 接口继承类
  select(): void;
}

class Button extends Control implements SelectableControl {
  select() { }
}

class TextBox extends Control {
  select() { }
}

// Error: Property 'state' is missing in type 'Image'.
class Image implements SelectableControl {
  select() { }
}

class Location {

}